1. Field of the Invention
The present invention relates generally to an apparatus and method for transmitting/receiving data in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for transmitting/receiving data with an increased reliability.
2. Description of the Related Art
It is impossible in practice to receive pure signals without signal distortion or noise. The influence of distortion or noise is more severe in a wired network than in a wireless network.
Accordingly, great amounts of time and energy have been expended toward minimizing the influence of distortion or noise involved with signal transmission and reception in a mobile communication system. The major method of reducing the effects of distortion and noise is through error control coding. Codes used for the error control coding are divided into memoryless codes and memory codes. The memoryless codes include a linear block code. The memory codes include a convolutional code and a turbo code. A channel encoder generates the code to include systematic bits and parity bits according to an error control coding technique. The turbo code is generally used for the error control coding. A systematic convolutional code also has systematic bits and parity bits.
The systematic bits are information bits transmitted from a transmitter to a receiver, and the parity bits are added at channel encoding to correct at decoding errors generated during transmission of the systematic bits. Even for an error control coded signal, burst errors in systematic bits or parity bits are difficult to correct for. The burst errors, which are often generated on a fading channel, can be prevented by interleaving. Interleaving is performed to distribute data having the same information to overcome the shortcoming of the error control coding.
An interleaved signal is mapped on a symbol basis in a digital modulator. The symbol mapping refers to designation of symbol positions in a two-dimensional plane (a symbol constellation) having an I channel along an X axis and a Q channel along a Y axis. It is determined according to a modulation scheme such as QPSK (Quadrature Phase Shift Keying), 8PSK, 16QAM (Quadrature Amplitude Modulation) and 64QAM. The symbol mapping is performed according to the number of bits in a modulation symbol and the values of the bits. The number of bits in a modulation symbol depends on a modulation scheme as listed in Table 1.
TABLE 1ModulationNumber of bits mappedQPSK28PSK316QAM464QAM6
Referring to Table 1, the number of bits in a modulation symbol increases as a modulation order increases. For example, one modulation symbol includes at least four bits in a modulation scheme having a modulation order equal to or greater than that of 16QAM. Bits are mapped to a modulation symbol in the symbol constellation.
Transmission reliability and symbol mapping according to modulation schemes will be described with reference to FIGS. 1, 2 and 3.
As illustrated in Table 1, one symbol in a general modulation scheme contains a plurality of coded bits and is represented by a point in a symbol constellation. The symbol constellation is divided into four macro regions (hereinafter, four quadrants), left, right, up and down according to positions along the X axis (I channel) and the Y axis (Q channel). Micro regions, namely, demodulation regions, are defined in each quadrant. Part of coded bits in a modulation symbol designate one of the four quadrants and the other coded bits designate a demodulation region or specific coordinates in the designated quadrant. The former coded bits are referred to as quadrant determining bits, and the latter coded bits are referred to as demodulation determining bits. Both I and Q channel values are positive in the first quadrant, I channel values are negative and Q channel values are positive in the second quadrant, both I and Q channel values are negative in the third quadrant, and I channel values are positive and Q channel values are negative in the fourth quadrant. The demodulation regions can be further divided according to other modulation schemes used.
During symbol transmission on a radio channel, most errors occur in demodulation regions in the same quadrant. Thus the error probability of quadrant determining bits is higher than that of demodulation determining bits. In other words, the quadrant determining bits have a relatively high reliability, whereas the demodulation determining bits have a relatively low reliability. In a modulation symbol, macro region determining bits are referred to as high-reliability bits, micro region determining bits are referred to as low-reliability bits, and any remaining bits are referred to as medium-reliability bits. These are bit reliabilities in the symbol.
FIGS. 1, 2 and 3 are diagrams illustrating symbol constellations representing the bit reliabilities of a symbol in 8PSK, 16QAM, and 64QAM, respectively.
Referring to FIG. 1, one modulation symbol contains three coded bits in 8PSK. The three coded bits determine the position of the symbol in the symbol constellation. Specifically, the first two bits indicate one of the four quadrants, and the remaining one bit indicates a point (coordinates) in the quadrant. For example, if a symbol has coded bits “011”, the quadrant determining bits “01” indicates the second quadrant and the demodulation determining bit “1” indicates a specific mapping point in the second quadrant.
Referring to FIG. 2, one modulation symbol occupies four coded bits in 16QAM. The first two quadrant determining bits indicate one of the four quadrants and the other two demodulation determining bits indicate a particular demodulation region in the quadrant. In FIG. 2, four demodulation regions are defined in each quadrant. For example, coded bits “1011” are mapped to a particular symbol indicated by the demodulation determining bits “11” in the second quadrant indicated by the quadrant determining bits “10”.
Referring to FIG. 3, one modulation symbol contains six coded bits in 64QAM. The first two quadrant determining bits indicate one of the four quadrants and the other four demodulation determining bits indicate a particular demodulation region in the quadrant. In the 64QAM symbol constellation, four main demodulation regions are defined in each quadrant and four sub-demodulation regions are defined in each main demodulation region. For example, coded bits “1101111” are mapped to an upper left symbol indicated by the sub-demodulation determining bits “11” in the upper left main demodulation determining bits “11” in the second quadrant indicated by the quadrant determining bits “10”. In FIG. 3, the main demodulation regions are marked with bold dotted lines and the sub-demodulation regions are marked with slender dotted lines.
Table 2 below illustrates symbol reliability patterns for 8PSK, 16QAM, and 64QAM.
TABLE 2ModulationSymbol reliability pattern8PSK[H, H, L]16QAM[H, H, L, L]64QAM[H, H, M, M, L, L]
In Table 2, H denotes a high reliability, M denotes a medium reliability, and L denotes a low reliability.
FIG. 4 is a block diagram of a transmitter in a typical HSDPA (High Speed Downlink Packet Access) mobile communication system. Referring to FIG. 4, the transmitter includes a channel encoder 430, an interleaver 440, and a modulator 450.
Upon input of N transport blocks from a data source 410, a CRC adder 420 adds CRC bits to each transport block. The channel encoder 430 encodes the CRC-attached N transport blocks at a code rate, for example; ½ or ¾. If the channel encoder 430 supports a plurality of coded rates through symbol puncturing or repetition with a mother code rate of ⅙ or ⅕, an operation for selecting one of the code rates is required. In the transmitter, the channel encoder 430 determines its code rate under the control of a controller 460.
While a rate matcher is not illustrated in FIGS. 4, 5 and 6, if required, it can be disposed between the channel encoder 430 and the interleaver 440. In this case, the rate matcher matches the data rate of the coded bits by repetition and puncturing when transport channel-multiplexing is needed, or if the number of the coded bits is different from that of bits to be transmitted through the air. The operation of the rate matcher will not be described hereinafter.
To minimize data loss caused by burst errors, the interleaver 440 interleaves the coded bits. The modulator 450 maps the interleaved bits to symbols in a modulation scheme determined by the controller 460. The controller 460 selects the code rate and the modulation scheme according to the current radio channel condition. To selectively use QPSK, 8PSK, 16QAM, and 64QAM according to the radio environment, the controller 460 supports AMCS (Adaptive Modulation and Coding). Though not shown, transmission data is spread with a Walsh code for channelization and spread with a PN (Pseudorandom Noise) code for identifying a BS.
The output of the channel encoder 430 is divided into systematic bits and parity bits that differ in priority. When transmission data has a particular error rate, it is better for decoding at a receiver to have errors in the parity bits than in the systematic bits because the systematic bits are real data and the parity bits are added for error correction at decoding, as described before.
Therefore there is a need for dealing with systematic bits and parity bits discriminately according to their priority levels in symbol mapping. To meet the need, SMP (Symbol Mapping based on Priority) is discussed for HSDPA (High Speed Downlinks Packet Access) standardization. SMP is a technique of combining the priorities of coded bits with reliabilities in a symbol. By SMP, high-priority coded bits and low-priority coded bits are mapped to high-reliability bit locations and low-priority bit locations, respectively in symbol mapping in order to reduce the probability of generating errors in relatively significant bits and thus improve reception performance.
FIG. 5 is a block diagram of a transmitter in a conventional HSDPA mobile communication system supporting SMP. The transmitter is characterized by mapping high-priority systematic bits to high-reliability bit locations in a symbol.
Referring to FIG. 5, a channel encoder 530 separately outputs systematic bits and parity bits. A first interleaver 540 and a second interleaver 550 separately interleave the systematic and parity bits. The first and second interleavers 540 and 550 are physically or logically separated to allow the coded bits to be mapped to symbols according to their priority levels. A PSC (Parallel-to-Serial Converter) 560 converts the interleaved systematic and parity bits to a serial bit stream according to the modulation scheme of a modulator 570 and the code rate of the channel encoder 530 under the control of a controller 580, taking into consideration that the number of systematic bits and parity bits change according to the code rate. The modulator 570 maps the serial bits to symbols. The symbols have a reliability pattern [H, H, L] in 8PSK, [H, H, L, L] in 16QAM, and [H, H, M, M, L, L] in 64QAM. Also shown are data source 510 and CRC adder 520.
Implementation of SMP in 16QAM will be described below.
When the code rate is ½ and the modulation scheme is 16QAM, the channel encoder 530 outputs systematic bits and parity bits which are equal in number, and each modulation symbol output from the modulator 570 has a reliability pattern [H, H, L, L]. Thus two systematic bits are mapped to a high reliability part (H), and two parity bits are mapped to a low reliability part (L).
When the code rate is ¾ and the modulation scheme is 16QAM, the channel encoder 530 outputs systematic bits and parity bits at a ratio of 3:1, and each modulation symbol output from the modulator 570 has a reliability pattern [H, H, L, L]. Thus two of three systematic bits are mapped to a high reliability part (H), and one systematic bits and one parity bit are mapped to a low reliability part (L).
The SMP technique implemented in the transmitter illustrated in FIG. 5 is disclosed in Korea Patent Application No. 2001-17925 filed by the present applicant, the contents of which are incorporated herein by reference.
FIG. 6 is a block diagram of a transmitter in a conventional HSDPA mobile communication system supporting CoRe (Constellation Rearrangement).
The CoRe technique is an advanced retransmission method under discussion, in which the reliabilities of bits in each symbol are averaged by rearranging a high-order modulation constellation at a retransmission. To achieve the purpose of the CoRe technique, the transmitter illustrated in FIG. 6 further includes a rearrangement controller 670. The same components as in the transmitter illustrated in FIG. 4 will not be described.
Referring to FIG. 6, the rearrangement controller 670 provides overall control in cooperation with a controller 660 to rearrange previously coded bits upon request for a retransmission from a receiver. A modulator 650 maps interleaved coded bits to symbols in a constellation which can be changed according to the number of transmission occurrences.
FIGS. 7A to 7D illustrate examples of constellations for initial transmission and retransmissions. Specifically, FIG. 7A illustrates a constellation for an initial transmission, FIG. 7B illustrates a constellation for a first retransmission, FIG. 7C illustrates a constellation for a second retransmission, and FIG. 7D illustrates a constellation for a third retransmission. For a fourth retransmission and afterwards, the constellations illustrated in FIGS. 7A to 7D are repeatedly used in sequence.
For example, coded bits “0011” are mapped to the upper right symbol in the first quadrant at an initial transmission as illustrated in FIG. 7A, then mapped to the lower left symbol in the third quadrant at a first retransmission as illustrated in FIG. 7B, then mapped to the lower left symbol in the first quadrant at a second retransmission as illustrated in FIG. 7C, and then mapped to the upper right symbol in the third quadrant at a third retransmission as illustrated in FIG. 7D. The first two bits “00” of the coded bits “0011” assumes high reliability (H) in the first quadrant at the initial transmission. “11” are high-reliability bits in the third quadrant at the first retransmission, “00” are high-reliability bits in the first quadrant at the second retransmission, and “11” are high-reliability bits in the third quadrant at the third retransmission. This constellation rearrangement effects averaging the reliabilities of the coded bits. While the constellations illustrated in FIGS. 7A to 7D are defined to 16QAM, the same constellation rearrangement may occur in all high-order modulations.
The SMP matches priority to reliability, and the CoRe averages reliabilities irrespective of priority. Yet the two techniques commonly utilize bits with different reliabilities within a symbol.
However, the SMP and CoRe techniques cannot coexist because symbol mapping irrespective of priority discords with priority-based processing of coded bits.